Runtime errors in PyTorch nodes

The following problems were discovered:

Undefined helper method and variable (#2)
- TrainModel.train calls self.make_image_tensor() which is not defined.
- Variable best_model may be used before assignment.
Dataset handling in inference (#3)
- PyTorchInferenceNode.inference assumes input dataset has a .dataset attribute, raising AttributeError for regular datasets.
FlattenDataset assumes .data (#4)
- FlattenDataset.flatten uses dataset.data which is not present in generic datasets.

These should be addressed to prevent runtime failures.